A spectrum of $\mathrm{\Delta }$-correlon quasiparticles is proposed to describe the excitation effect on electron correlation. The proposed quasiparticle description is based on the comparison of the pair-correlation functions of the excited and ground states at the special coalescence points of the configurational space. The complex form of the $\mathrm{\Delta }$-correlon wave function allows us to naturally separate regions with the excitation suppression of correlation (ESC $\mathrm{\Delta }$-correlon), where correlation is stronger in the ground state, from those with the excitation enhancement of correlation (EEC $\mathrm{\Delta }$-correlon), where correlation is stronger in the excited state. The proposed $\mathrm{\Delta }$-correlon approach is applied to characterize the important single excitations with the competition of “anticorrelation squeezing” and “enhanced-correlation spreading” of the two-electron distribution. The former feature characterizes single valence excitations of the ionic nature and this leads to the prevailing of the ESC effect in this type of excitations. In turn, the enhanced-correlation spreading characterizes single Rydberg excitations, which leads to the prevailing of the EEC effect in this type of excitations. The proposed $\mathrm{\Delta }$-correlon quasiparticles are studied both analytically and numerically. The analytical expressions for the on-top correlation functions are obtained with the two-determinantal two-electron model of the active site of an excitation. The two-dimensional contour plots of the ESC and EEC $\mathrm{\Delta }$-correlons obtained for the lowest vertical valence ionic and Rydberg excitations in the prototype molecules ${\mathrm{H}}_2, {\mathrm{N}}_2$, and ${\mathrm{C}}_2{\mathrm{H}}_4$ from the highly correlated wave functions confirm, in general, the trends in the relative correlation strength deduced from the model.

• Received 7 May 2021
• Accepted 21 July 2021

DOI:https://doi.org/10.1103/PhysRevA.104.022804